Control for a tankless water heater used with a solar water heating system

ABSTRACT

A tankless water heating auxiliary system for a solar water heating system, includes a solar collector; a tankless water heater auxiliary system; an insulated water storage tank storing the potable water; a heat exchange system for heating stored water; and piping for connecting the collector, the storage tank and the heat exchanger in fluid communication. A first sensor is connected to and located adjacent the storage tank for sensing the temperature of the stored water at an outlet of the tank. A method for controlling initiation of heating in a tankless water heater auxiliary system, includes monitoring operation of a tankless water heater; measuring water flow using a water flow sensor to determine if water flow rate exceeds a use determined flow rate; implementing a control time delay into the tankless water heater to purge water from the heater and sense the inlet water supply temperature; measuring the water temperature using a heat exchanger outgoing thermistor; comparing the temperature measured by the thermistor to a predetermined temperature; and initiating a combustion sequence if the temperature measured by the outgoing themistor is less than the predetermined temperature.

BACKGROUND OF THE DISCLOSURE

The disclosure relates to a control for a tankless water heater servingas an auxiliary heater for a solar water heating system.

Solar water heating systems are made up of components that collect solarenergy, transfer the energy to the potable water via a heat exchanger,store the thermal energy, control system operations, deliver hot waterwhere it is needed, and protect the system against freezing. Componentscan be combined in a variety of ways, but these six basic functions mustbe met, although some systems are simple enough that passive physicsprovide the control and motive forces to drive the fluids and heattransfer in the system. The systems also typically include an auxiliaryheating system for when the solar heating is insufficient to meet hotwater usage.

Solar water heating systems can generally be described using thefollowing four terms: direct or indirect and passive or active. Directsystems heat the potable water directly through solar exposure. Thesesystems are typically used in regions with little freeze risk since thepotable water is used as the heat transfer fluid and therefore exposedto temperature outside of conditioned space. Indirect systems use asecondary heat transfer fluid to collect and transfer the solar energyto the potable water. The heat transfer fluids used are typically freezeresistant and these systems are better suited for cooler climates.Passive systems use the differential density created by the thermalgradient of the water or heat transfer fluid to move fluids in thesystem and accumulate heated potable water for usage. Active systemsemploy pumps to circulate the water or heat transfer fluid for thepurpose of heat exchange. Several different system configurations arecurrently used based on these concepts. Solar water heating systems arecapable of heating water to high temperatures, commonly up to 160° F. orhigher. The system typically uses a thermostatic mixing valve (TMV) totemper the water to a selected level such as 110° F. to 140° F., toreduce the burn risk of the water as well as increase the energy storagecapacity of the water storage tank.

Solar water heating systems typically employ one or more water storagetanks to contain the solar energy collected over the sunlit hours. Sincethe solar energy resource is not synchronized with the hot water usage,the energy storage function is an essential part of the system. Duringperiods when the solar resource is insufficient to meet water usage,such as during cloudy weather conditions or during high hot waterdemand, an auxiliary heating system is needed to supply hot water.Several options exist for electric or gas auxiliary heating systems.Typically, a conventional electric or gas storage water heater can becombined in series with the solar water storage tank. Since thisapproach requires two storage tanks, significant floor space is neededto accommodate the arrangement. Single tank electric systems existemploying a large single tank, which thermostatically operate aresistance heater to heat the upper portion of the tank while the solarsystem provides heat to the lower portion of the tank. A gas tanklesswater heater can serve as a more compact gas auxiliary heater coupledwith a solar water storage tank. The tankless water heater can either bemounted to the storage tank or in close proximity to the tank.

Tankless water heaters, also known as instantaneous or on demand waterheaters, serve as inline heater exchangers which raise supplied waterfrom the local supply temperature to a user selected set temperaturelevel, usually in the range from 110° F. to 140° F. Tankless systemshave negligible water storage capacity, typically less than 2 gallons.Heating is accomplished using high output, fixed level or variable gasburners, ranging from 50,000 BTU/hr to 200,000 BTU/hr, or electricresistance heaters ranging from 5,000 to 9,500 watts. Gas systems aremost common due to the high energy requirement needed to create thedesired temperature rise at flow rates needed to service a typicalresidence or light commercial application.

Tankless water heaters have previously been coupled with solar heatedstorage water tanks, serving as an auxiliary. The arrangement can beconfigured with the solar heated storage tank supplying water to aheated water circulation loop, which circulates heated water to usagelocations with a continuously running or thermostatically controlledpump. The temperature of the circulation loop is maintained by theoperation of the tankless unit, while the solar heated supply waterfeeds the system with water that requires a minimal temperature liftprior to usage. This configuration typically utilizes a small, 2- to5-gallon, storage tank near the usage location to level the outlet watertemperature. While this system supplies a consistent and immediatesupply of heated water, it consumes excessive energy to maintain thecirculation loop temperature and requires additional equipment at thetime of installation.

The typical tankless water heater operation control method, whether gasor electric, senses the initiation of inlet water flow triggering aheating even independent of the upcoming water temperature to the unit.When cold water is being supplied to the tankless unit, this methodprovides the best heating performance with an appropriate level ofefficiency. The conventional tankless operation control does not makeprovision for incoming water supply, previously heated at or above theset tankless units point, and as a result needless heating evens areinitiated consuming unneeded energy and increasing the number of heatingsystem operating cycles contributing to a shortened life of theequipment.

A conventional way of providing an on-demand supply of hot water tovarious plumbing fixtures is to use a tankless or “instantaneous” waterheater in which water is flowed through a high heat input heatexchanger, without appreciable water storage capacity, so as to provideonly as much hot water as needed by the open fixture(s). Where higherhot water flow rates than the instantaneous water heater can provide atthe desired heated temperature are required, it has been conventionalpractice to connect a storage tank to the instantaneous water heater, inseries, to supplement the hot water delivery capability of theinstantaneous water heater with pre-heated storage tank water.

Thus, it is considered desirable to provide an improved tankless waterheater control when used as an auxiliary with a solar water heatingsystem, which overcomes the above-mentioned deficiencies and otherswhile providing a better and more advantageous overall result.

SUMMARY OF THE DISCLOSURE

According to one aspect of the disclosure, a tankless water heatingauxiliary system for a solar water heating system, includes a solarcollector; a tankless water heater auxiliary system; an insulated waterstorage tank storing potable water; a heat exchange system for heatingstored water; piping for connecting the collector, the storage tank andheat exchanger in fluid communication; and a first sensor connected toand located adjacent the storage tank for sensing the temperature of thestored water at an outlet of the tank.

According to another aspect of the disclosure, a method for controllinginitiation of heating in a tankless water heater auxiliary system,includes: monitoring operation of a tankless water heater; measuringwater flow using a water flow sensor to determine if water flow rateexceeds a use determined flow rate; implementing a control time delayinto the tankless water heater to purge water from the heater and sensethe inlet water supply temperature; measuring the water temperatureusing a heat exchanger outgoing thermistor; comparing the temperaturemeasured by the thermistor to a predetermined temperature; andinitiating a combustion sequence if the temperature measured by theoutgoing themistor is less than the predetermined temperature.

Another aspect of the disclosure is a solar water heater systememploying a tankless, (instantaneous) inline heater as an auxiliarywhich is controlled using a sensor configuration which determines thestored water temperature as a condition for initiating the powering orcombustion sequence for the auxiliary heater. The sensor configurationeliminates needless cycling of auxiliary systems and reduces energyconsumption.

Another aspect of the disclosure is to place an auxiliary control sensorat the outlet or top of the solar water storage tank which senses thewater delivered to the tankless auxiliary and prevent needless poweringof the auxiliary power system. This solution would reduce energyconsumption and reduce the number of operation cycles of the auxiliary,improving reliability.

Another aspect of the disclosure is reduced energy consumption in therange of $20-$40 annually and a reduction in the number of operationcycles, improving system reliability.

Another aspect of the disclosure is a stored/delivered water temperaturesensor which determines heating requirements for the series combinationof a heat pump water heater storage tank and an instantaneous waterheater.

Still other aspects of the disclosure will become apparent after areading and understanding of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the solar water heater tank and tanklessauxiliary system in accordance with a preferred embodiment of thedisclosure;

FIG. 2 is a flow chart illustrating the sequence of steps of thecombustion sequence of the tankless auxiliary;

FIG. 3 is a control board for the system of FIG. 1; and

FIG. 4 is a flow chart illustrating a sequence of steps of thecombustion sequence of the tankless auxiliary in accordance with anotherembodiment of the disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a solar water heater supply system 10 in accordancewith a preferred embodiment of the present disclosure is shown.

The system 10 includes a solar collector 12 which may be of conventionaldesign which collects and provides heat to a heat transfer fluid thatthen transfers the heat from the collector 12 to water contained in aconventional insulated water heater storage tank 14 having a cold waterinlet 16 near a bottom 17 of the tank and a hot water outlet 18 near atop 19 of the tank. A thermostatic mixing valve 20 is provided at a hotwater outlet pipe 22 which mixes cold water from a cold water supplyline 26 with the hot water from hot water line 22 to provide the userwith water at the user selected water temperature.

Such mixing valves 20 are typically used in solar hot water heatingsystems because it is desirable to store water in the tank attemperatures which may be higher than that desired for use by the user,for increased energy storage. This is because solar energy is onlyavailable periodically over the course of a day, e.g., during daylighthours, and the intensity of the sunlight varies even during such hours,so in order to take the fullest advantage of the energy when it isavailable, the water in the heater storage tank may be allowed to reacha higher temperature than that desired by the user when turning on thehot water. For example, the maximum temperature for the water in thetank may be pre-set at a value on the order of 160° F., while the usermay prefer a setting in the 110° F.-120° F. range. The mixing valveblends cold water as needed with the hot water to properly limit thetemperature of the water from the water heater system to the temperatureselected by the user, effectively increasing stored hot water capacity.Mixing valve 20 may be any one of many commercially availablethermostatic mixing valves.

Referring to FIG. 1, cold water enters the tank 14 via cold water inlet16. Cold water exits the tank via outlet 28 and enters pipe 30 and flowsto the solar collector panel 12. The collector warms the water whichthen passes through warm water outlet pipe 31 to the tank 14 and entersthe tank via inlet 32. Warm or hot water exits the tank via outlet 18and pipe 22 to the tankless water heater 24 via inlet 54. The tank 14 islocated in a conditioned space and is vertically close to the collectorto reduce head pressure.

A control system determines when sufficient solar energy is availableand controls system operation accordingly. A controller 77 is operativevia an electrical wire connection to push fluid through the system. Inthe illustrative embodiment, the control system includes sensor 33, athermistor sensor which can be band clamped to a copper fluid line atthe top of the tank 14 to sense the temperature of the fluid in the lineat that point. Alternatively, sensor 33 can be mounted inside the tank.Sensor 33 is connected via wire 35 to tankless heater 24 and controller77. The power to energize the system under normal operating conditionsis provided by an external power source such as a 120 volt ac powersupply 34 (FIG. 3) from a utility company, or an integrated power sourcesuch as a photovoltaic cell collector. A reserve power supply, such as abattery, is preferably included to provide power to evacuate the heattransfer fluid from the system in the event of a failure of the primarypower supply.

Pipe 31 extends out from a solar collection panel 12 which absorbs solarheat and is drawn to the hot water reservoir tank 14 via inlet 32. Thewarm water is removed from the warmest area of the upper portion 19 ofthe water tank through a warm water pipe 22 attached to upper portion ofthe hot water reservoir tank.

Hot water demand exceeding the stored volume of the tank is supplied bythe tankless gas water heater. Alternatively, an electric pump operationcould also be easily configured to provide electric only operation ifgas prices become uncompetitive during all or a seasonal portion of theyear. In this scenario, tankless units can simply be disabled.

The tankless, instantaneous inline heater 24 acts as an auxiliary and iscontrolled using a sensor configuration which determines the sortedwater temperature, based on a user selected temperature level, as acondition for initiating the powering or combustion sequence for theauxiliary heater. The sensing eliminates needless cycling of auxiliarysystems and reduced energy consumption.

The tankless water heater serves as an inline heater exchanger whichraises supplied water from the local supply temperature to a userselected set temperature level, usually in the range from 110° F. to140° F. The tankless system itself has negligible water storagecapacity, typically less than 2 gallons. Heating is accomplished usinghigh output, fixed level or variable gas burners, ranging from 50,000BTU/hr to 200,000 BTU/hr, or electric resistance heaters ranging from5,000 to 9,500 watts. Gas systems are preferred due to the high energyrequirement needed to create the desired temperature rise at flow ratesneeded to service a typical residence or light commercial application.

Existing tankless water heater operation control methods, whether gas orelectric, sense the initiation of inlet water flow triggering a heatingeven independent of the upcoming water temperature to the unit. Whencold water is being supplied to the tankless unit, this method providesthe best heating performance with an appropriate level of efficiency.Existing tankless operation controls do not make provision for incomingwater supply, previously heated at or above the set tankless unitspoint, and as a result needless heating evens are initiated consumingunneeded energy and increasing the number of heating system operatingcycles contributing to a shortened life of the equipment.

Referring now to FIG. 3, tankless auxiliary water heater 24 carries outoperation of a heat exchanger 42 which has a burner 44. A remote control46 can have various controlling elements or indicators, such as a hotwater tension switch, a heating switch, a “burner on” indication light,a driving switch, a water temperature setter and a fluctuation switchand is connected to the system via a wire 48.

Referring to FIG. 2, a logic diagram of the tankless operation sequenceis as follows. An inquiry 50 is made in which a water flow sensor 52measures whether the flow from water inlet 54 is more than 2.4 litersper minute. If the answer is YES, additional remote tank sensor 33 onthe tank 14 then measures the water temperature at the tank. An inquiryis made whether the temperature is less than 120° F. If the answer toinquiry 56 is NO, then the water flow sensor again measures water flow.If the answer is YES, then combustion fan 55 is turned on at step 58.This saves energy and starts up the flow event in the tankless heater.Thus, the tank is regulated even if the water is not 160° F. An inquiry60 is made if the fan rotation is detected to be NORMAL. If the answeris YES, an initial check is made if normal at step 62. If YES, theignition is turned on at step 64. Then, referring to step 72, thevarious solenoid valves are operated. Main solenoid valve 66, valves 67,68, 69 and modulating valve 70 are operated at step 72 and allow gasinto the system via inlet 73. An inquiry 74 is made as to whether theflame at burner 44 is detected to be greater than 1 μA. If YES, then hotwater leaves the heater via outlet 76.

In a high temperature state, the solar hot water temperature detectedwith the temperature sensor is higher than the hot-water-supply presettemperature or undesired water temperature setter of thehot-water-supply remote control, a combustion signal is sent to thehot-water-supply controller to prevent combustion movement in the hotwater supply system.

Referring to FIG. 3, a control board 77 for the tankless auxiliary waterheater 24 of FIG. 1 is shown.

A heat exchanger outgoing thermistor 80 is used to measure hot watertemperature at an outlet of heat exchanger 42. If water temperaturereaches a predetermined limit, gas supply from gas inlet 73 is stopped.An overheat switch 82 is situated on the heat exchanger 42 to preventgas supply to the heater such as when water temperature reaches apredetermined limit, such as 97° C. for a predetermined amount of time.A thermal fuse 84 is situated on the heat exchanger 42 to preventelectrical power supply to the heater such as when the temperatureexceeds a predetermined limit such as 129° C.

The gas tankless control monitors the stored water temperature as acondition for initiating a heating operation. The control logic is thesame as used by heat exchanger outgoing thermistor 80 with the additionof a logical “and” check of the remote temperature sensing thermistor 33such as shown in FIG. 2.

The temperature of the outgoing hot water is constantly monitored by thewater temperature thermistor 33 located near the outlet of the tank. Ifthe outgoing water temperature reaches a temperature above the presettemperature such as 5° F. above the preset temperature, the burner 44will automatically go out or be directed not to start based on theremote water temperature thermistor 33 sensed value. The temperaturedifference calibration is to be a user or factory input. The burner 44will only ignite again once the outgoing hot water temperature fallsbelow the preset temperature.

Sensing probe or thermistor 33 is placed directly on the storage tank14, preferably adjacent to or on the top 19 of the tank. Alternately,sensor 33 can be placed inside the tank. Placement of the sensordirectly on or inside the tank provides direct sensing of the intendedsupply water and negates the effect of water which has cooled in thepiping between the storage tank and the instantaneous heater unit'sinboard sensors. The “cool” volume of water in the piping may create abrief powering event which is quickly terminated when the “hot” storedwater arrives from the solar storage tank. The remote sensing can beused in parallel with the existing heat exchanger sensor 80, whichdetected the heat exchanger temperature in addition to the existence offlow as a requirement for the initiation of heating power.

Gas utilities are always seeking energy efficient water heatingsolutions in order to receive “green” credits as well as an increasingconsumer demand for these products. US gas/water/heater sales are 5million units annually.

By placing the sensor 33 at the outlet or top of the solar water storagetank, needless powering of the auxiliary power system is prevented. Thisconfiguration reduces energy consumption and reduces the number ofoperation cycles of the auxiliary, improving reliability.

Referring to FIG. 4, another embodiment is described using a flow chartdiagram. In this embodiment, a tankless auxiliary control mode allowsthe heating or combustion sequence to occur or be triggered by comparingthe heat exchanger outgoing thermistor 80 temperature level relative toa user selected temperature level.

The response to this control input will only be active during a flowevent. A time delay after the detection of water flow is needed in orderto purge the water in the system and sense the inlet supply watertemperature.

Specifically, referring now to FIG. 4, at step 90 the tankless auxiliaryoperation is monitored as previously described for FIGS. 1-3. Proceedingto inquiry 92, water flow sensor 52 measures the water flow rate fromthe water inlet 54. An inquiry is made as to whether the water flowdetected is greater than a set flow level (e.g. such as 2.4liters/minute). If the answer is YES, then step 94 is initiated, inwhich a delay logic is added for an “x” time interval, in which “x” timecan be preset by the user. The time delay is needed after the detectionof water flow in step 92 to purge the water in the system and sense theinlet supply water temperature.

Proceeding to inquiry 96, the heat exchanger outgoing or exit thermistor80 temperature level is compared to a preset user temperature level, andan inquiry is made as to whether the heat exchanger outgoing thermistortemperature level is less than the set level. If the answer is YES, thenthe heating event or combustion logic is initiated as step 98, which issimilar to the steps 58-74 described in FIG. 2.

The disclosure has been described with reference to the preferredembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations.

What is claimed is:
 1. A tankless water heating auxiliary system for asolar water heating system, comprising: a solar collector; a tanklesswater heater auxiliary system; an insulated water storage tank storingpotable water; a heat exchange system for heating stored water; pipingfor connecting said collector, said storage tank and said heat exchangerin fluid communication; and a first sensor connected to and locatedadjacent said storage tank for sensing the temperature of the storedwater at an outlet of said tank.
 2. The tankless water heating auxiliarysystem of claim 1, further comprising a controller connected to saidfirst sensor.
 3. The tankless water heating auxiliary system of claim 2,wherein when said first sensor detects whether the water temperature insaid storage tank is greater than or less than 120° F.
 4. The tanklesswater heating auxiliary system of claim 3, wherein said first sensor isplaced on a top surface of said tank.
 5. The tankless water heatingauxiliary system of claim 3, wherein said first sensor is placed insideof said tank.
 6. The tankless water heating auxiliary system of claim 4,wherein said first sensor is connected to said tankless heater via wire.7. The tankless water heating auxiliary system of claim 6, wherein saidfirst sensor is operative to detect a temperature greater than apredetermined maximum reference water temperature.
 8. The tankless waterheating auxiliary system of claim 7, wherein when said first sensordetects a temperature below a predetermined maximum referencetemperature, said burner is activated.
 9. The tankless water heatingauxiliary system of claim 7, wherein when first sensor detects atemperature above 120° F., said burner is not activated.
 10. A methodfor controlling initiation of heating in a tankless water heaterauxiliary system, comprising: monitoring operation of a tankless waterheater; measuring water flow using a water flow sensor to determine ifwater flow rate exceeds a use determined flow rate; implementing acontrol time delay into the tankless water heater to purge water fromthe heater and sense the inlet water supply temperature; measuring thewater temperature using a heat exchanger outgoing thermistor; comparingthe temperature measured by said thermistor to a predeterminedtemperature; and initiating a combustion sequence if the temperaturemeasured by said outgoing themistor is less than said predeterminedtemperature.